This invention relates to selective detection of specific compounds and in particular to the concentrating of vapors, especially explosives vapors, in air samples and airborne samples acquired from surfaces.
Detection of explosives carried by persons or concealed in buildings, airplanes, cars or other locations can be vital to prevention of injuries and damage to property. However, detection by direct searching is quite costly and time-consuming, can at times be dangerous, and can also be susceptible to error. Thus, it is desirable to detect explosives somewhat indirectly, as by their presence in very small amounts of vapors in air or other gases which have been in contact with explosives in solid or liquid form.
To be effective, devices for detecting vapors of selected compounds such as explosives in air must fulfill several requirements. They must, of course, be reliable. Also, they need to be highly sensitive in order to detect the minute quantities (parts per quadrillion (10.sup.15) or less) present in vapors and which in turn may indicate the presence of much larger quantities of the compounds. It is essential that explosives detection systems be very selective so as to prevent or minimize false alarms which would result from detection of compounds which are not explosives, and yet be highly reliable so that no explosives present are overlooked or not detected. In certain applications, such as screening persons for possession of explosives, detectors must operate rapidly--they must determine, essentially in real time, whether explosives are present--and they should also be as non-intrusive as possible. For many situations, it is important that the detector identify the specific explosive detected. Other characteristics which may be important in an explosives detector are that it be portable, rugged, and able to function in harsh environments.
Various systems are known for detecting specific compounds such as explosives, but none have provided the combination of selectivity, sensitivity, reliability, and rapid response needed for an effective and reliable detector. Systems such as electron capture detectors, mass spectrometers, ion mobility spectrometers, and nitric oxide chemiluminescence analyzers have been employed for detecting explosives, as have certain animals (notably dogs). The systems may perform satisfactorily if provided with high or moderate levels of certain explosives vapors and if allowed ample time for analysis. However, they generally are slow and also fail to provide the selectivity to distinguish explosives from various other compounds, particularly nitrogen-containing compounds, whose vapors may be present along with the explosives. The selectivity of such systems decreases as the amount of specific vapors available for detection decreases and is a significant drawback in detection of low levels of explosives. Attempts to concentrate vapors have proven time-consuming and have resulted in premature breakdown of the explosives vapors during their subsequent release for selective analysis. As a result, non-explosives such as halogenated solvents, nitrosamines, perfumes, nitrogen oxides (NO.sub.x), and phthalates interfere with, and may give false readings instead of, accurate detection of explosives.
It is an object of the invention to provide apparatus for concentrating vapors of specific compounds and for releasing the vapors without decomposing them.
It is an object of the invention to provide apparatus for concentrating and releasing vapors of specific compounds without trapping oxides of nitrogen in any significant amount.
It is an object of the invention to provide apparatus for rapidly concentrating vapors received in a hydrogen carrier gas from a collector and for quickly releasing the vapors as a focussed, small volume injection to a gas chromatograph.
It is a particular object of the invention to provide a two-stage vapor concentrator for successively concentrating vapors of compounds such as explosives and for injecting the concentrated vapors into a gas chromatograph.
It is an object of the invention to provide vapor concentration apparatus which can rapidly concentrate and desorb explosives vapors to facilitate high speed, highly selective, highly sensitive detection of explosives.